This invention relates to a control system for a batch-type pyrolysis device of the type used for volatilizing and burning organic material from a metal part to which the organic material is bonded. Incineration occurs in a zone adjacent to the device's main chamber in which the material is volatilized, but prior art devices are unable to provide a smokeless discharge into the atmosphere under normal conditions of commercial operation. By "discharge" we refer to combustion products issuing from the furnace's stack, and by "smokeless" we refer to the discharge being substantially clear to the naked eye, that is, permeable to light in the visible wavelength range.
Such an incineration zone is typically provided by an afterburner chamber in which an afterburner, positioned downstream of the device's main chamber in which pyrolysis occurs, burns the volatilized organic material (referred to herein as "vapor"). The remaining metal part is reclaimed for reuse because the cost of reclamation is less than that of making the metal part anew. Such reclamation by pyrolysis has evolved into a subindustry of considerable economic significance not only because pyrolysis is cost-effective, but also because incineration of the vapor of polymeric materials which are not economically recyclable, conveniently and beneficially disposes of them.
The vapor to be incinerated is generated when mounting means for engines and electric motors (collectively referred to as "motor mounts"), and similar steel parts bonded to rubber; or, copper-containing electrical parts such as armatures, stators, transformers and the like; or, painted ferrous or non-ferrous steel parts; or, metallic bodies of arbitrary shape which are coated with, or bonded to polymeric materials (referred to herein as "polymer-bonded metal parts"), are to be pyrolized in a pyrolysis furnace.
Polymeric materials to be disassociated from metal parts are such materials as are commonly bonded to a metal substrate or matrix and include natural and synthetic elastomers; for example, natural rubber and synthetic rubber which are polymers of dienes; silicones which are polymers of siloxanes and the like; and, natural and synthetic resinous materials including natural shellac and synthetic plastics such as phenolics and acrylics, particularly paints. The difficulty of incinerating the materials smokelessly varies; silicones do not burn smokelessly, but silicone-free rubbers and paints can now be reliably and economically incinerated, and smokelessly.
The foregoing polymeric materials are to be separated from the metal matrix to which they are bonded without melting the metal, and preferably, in most instances, without causing warpage or other undesirable deformation of residual metal matrix. It is self-evident that such separation may be effected by directly incinerating the polymeric materials, as is typically done in an incinerator for waste, but it is equally self-evident that the requirement of incineration without damaging the metal parts will not be met. Of course, damage to the parts can be minimized if only a few parts are incinerated together, but this method is undesirable because it does not lend itself to reclaiming a large enough mass of parts to be economical.
This invention is specifically directed to burning relatively large loads of metal parts combined with silicone-free polymers ("burnables") which are to be incinerated smokelessly in a relatively small main chamber, that is, with a relatively high ratio of load (lb)/volume (ft.sup.3), referred to as the load/volume ratio. Such loads contain from 0.1 lb of burnables per lb of metal, to 2 lb burnables/lb metal, and are referred to as "high-polymer" loads in contrast to conventional loads which contain less than 0.1 lb burnables/lb of metal.
The term "pyrolysis oven" has been used in the art to indicate that there is no incineration of organic material on the metal parts within the oven's main chamber. The material is simply volatilized (or vaporized) without being burned in the oven's main chamber. The vapors are then burned in the afterburner chamber, but not before they have exercised the opportunity to plug water spray nozzles used to keep the volatilization of burnables in the main chamber under control. Such operation of a "pyrolysis oven", where there is no fire in the main chamber, is supposed to clearly distinguish its function, from that of a "pyrolysis furnace" in which there is. Nevertheless, the terms are often misused or interchanged, particularly in relation to devices using an afterburner in an afterburner chamber of the furnace, with no thought given as to the significance of where the fire is maintained.
The desirability of a smokeless discharge from the stack of a pyrolysis furnace cannot be overemphasized. It is common practice to operate such a furnace during the day in such a manner that the smoky discharge is not too objectionable, reserving such operation for darkness. More responsible operators provide plural afterburners in series to make sure that as complete combustion as possible is obtained. The seriousness of the problem is such that even in a drying furnace where a relatively small amount of contaminating oil is being burned, plural burners are used, as disclosed in U.S. Pat. Nos. 3,767,179 and 3,839,086 to Larson.
Where the weight ratio (weight of burnables to be burned): (weight of metal) is relatively high, that is in the range from 0.1:1 to 2:1, a manufacturer of a prior art furnace advises against burning such loads. Attempts to burn even a small load result not only in the discharge of a highly noticeable stack gas, but also in the severe fouling of the furnace's main chamber, the controls, and, most important, of the water nozzles upon which the safe operation of the furnace is critically dependent.
An attempt to deal with the problem of fouling water spray nozzles is found in U.S. Pat. No. 4,557,203 to Mainord who uses a first sensor in the stack downstream of the afterburner to actuate a first set of nozzles; and a second sensor in the main chamber to actuate a second set of nozzles.
A highly successful control system for a pyrolysis furnace in which incineration occurs in the main chamber and also downstream of the afterburner is provided in copending application Ser. No. 822,022 filed Jan. 26, 1986 now U.S. Pat. No. 4,649,834. This system uses a single water spray system controlled by a first thermocouple in the main chamber (main chamber TC), and another (third) thermocouple (throat TC) in the vent passage ("throat") connecting the main chamber to the afterburner chamber. A second thermocouple in the stack (stack TC) controls only on/off or attenuated operation of the main burner. The effectiveness of this control system, in large measure, derives from the difference in temperatures sensed by the first and third thermocouples.
It was found that with high-polymer loads with the above-specified burnables content, the rise of temperature in the initial portion of the burn cycle was often uncontrollable, resulting in dense smoke and excessive temperatures in the main chamber. This occurred even when the furnace is constructed with a "vent number" greater than 0.003/ft found to be critical for normal operation. The vent number is computed by dividing the area of the vent (throat, ft.sup.2) by the volume of the main chamber (ft.sup.3).
It was not then realized that the sensitivity of the throat thermocouple is such that, a controlled rate at which the temperature of the load is raised ("ramped") can control a burn so effectively as to provide a smokeless stack even when burning a load of high-polymer parts. And most important, that the entire burn cycle may be controlled with the throat TC, so that the main chamber TC and the stack TC are used to provide redundant safety of operation.
This invention is specifically directed to a pyrolysis furnace with a single afterburner in an afterburner chamber, in which furnace a fire is sustained in the furnace's main chamber, while the temperature is ramped to preselected progressively higher set-points with intervening soak intervals, after which the temperature is maintained constant during a final load-cleaning burn (referred to as the "final soak period"). The surprising result is that there is essentially no visible smoke issuing from the stack, and no runaway increase of temperature.
A charge of metal parts on a cart is charged to the main chamber, the charge is brought up to ignition temperature at a predetermined rate which is controlled by a programmable control means, ignited, and the fire sustained under controlled "ramp and soak" conditions until the charge is burned out.
It is known that heating of the metal parts to 700.degree.-800.degree. F. in an enclosure with limited air intake will char or degrade all known combustible contaminants without ignition if the percentage of contaminants is less than about 2% by weight ("wt") of the parts. However, we are concerned with igniting much higher amounts of combustibles in the range from about 10% by wt of the load in the charge to about twice the weight of the load, or even more, and it is critical that the ignition result in an essentialy smokeless stack.
It is unnecessary to point out that, when operating under near-explosive conditions and a very small misstep can set off an explosion, a smokelesss stack may be an exiguous consideration. But any control system which provides a smokeless stack, yet prevents such an explosion from being set off, acquires great merit. In other words, a smokeless furnace must be operated with no sacrifice of safety. Our invention does so.
A reclamation oven with a control system for preventing fires and explosions and thus controlling excess temperature within it, is disclosed in U.S. Pat. No. 4,270,898 to Kelly. The fire and explosion control method senses a fire situation before it occurs, and keeps the fire from happening by instituting a timely extinguishing system. A thermocouple is installed in the exhaust, downstream from the afterburner, and when the temperature exceeds a preset temperature, a signal from the thermocouple actuates an automatic valve assembly to open it and spray water onto the too-hot parts in the main chamber. When the parts cool sufficiently, the valve assembly closes. The system prevents fires and explosions and thus controls excess temperatures. The main burner is not shut off when the water spray comes on, though the main burner goes off when the oven reaches the set-point temperature, nor is the average temperature above the metal parts in the oven's main chamber (referred to as the "ambient temperature" in the main chamber) monitored. The prior art system in which a fire in the main chamber is prevented, is wholly ineffective to minimize the smoke issuing from the stack, and as Mainord states, is responsible for plugging water nozzles. It is quite unlike our system in which a fire is maintained under conditions imposed by alternately ramping temperature, then maintaining it constant ("soaking").
Another system relating to incineration of unwanted organic material such as oil associated with metal parts, particularly scrap or swarf, is disclosed in U.S. Pat. No. 3,705,711 to Seelandt et al. Only as much air and fuel as is required to fuel the main burner, is burned to minimize oxidation of the metal parts and to minimize the risk of explosion. It is evident that such conditions of operation are calculated to generate more smoke because of incomplete combustion, not minimize the smoke generated. Control is provided by limiting the amount of combustion air to the main chamber when a preset pressure is exceeded. It is suggested that the temperature within the drum may first be lowered by throttling back the main oil burner or by stopping the feeding of metal scrap into the dryer drum. When the main burner output is reduced to its lower limit and the temperature within the drum is still too high, a water spray may be actuated. Should the spray be insufficient to lower the temperature, the feeding of the scrap into the drum is reduced or stopped. The problem is that the time period required for these operations is much longer than that permitted by conditions under which an explosion occurs because of ignition of the built-up vapor. As a result, such a system is wholly unsatisfactory under the conditions of operation of a pyrolysis furnace.
The control system of our invention allows the safe and smokeless burn of a high-polymer load by controlling a single stage of the burn cycle, namely the ramping stage. Control of the temperature in the throat to track the ramp and soak profile with an intermittent water spray, is the only essential and critical requirement of our single-stage system. No prior art control system for a pyrolysis furnace recognized the importance of a controlled temperature ramp, or ascribed any significance to, or suspected a correlation between the ramp controlled by a throat TC in cooperation with a PC, and a smokeless discharge.
The efficacy of our system is predicated on the discovery that the throat is the critical location in the furnace, at which single location, it is critical that we control the rate at which the temperature in the main chamber is increased. Such control serves a double-barreled purpose--it provides a safe burn, controlled with a water spray, and it provides a smokeless stack gas.